[under bump metallurgic layer]

ABSTRACT

An under bump metallurgic (UBM) layer which is adapted for a chip is disclosed. The UMM layer alleviate the loss of electromigration resulting from current crowing effect at the corner of UBM layer near the transmission line. By increasing the thickness of the UBM layer at the particular region which is close to the transmission line, losses of the UBM layer due to electromigration can be compensated. The life time of the chip is, therefore, enhanced.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Taiwan applicationserial no. 92121599, filed on Aug. 07, 2003.

BACKGROUND OF INVENTION

1. Field of the Invention

The present invention relates to an under bump metallurgic (UBM) layer,and more particularly to an improved structure of a UBM layer with lesselectromigration loss.

2. Description of the Related Art

For semiconductor industry, the manufacturing of integrated circuitscomprises three main stages: fabrication of wafers, fabrication ofintegrated circuits and package of the integrated circuits. Dies areobtained by sawing wafers on which circuits are designed andmanufactured. The dies connect external circuits via bonding padsthereon. The dies then are packaged so that the dies are protected fromthe influences of moisture, heat and noises. For the dies with packages,the package of the die acts as the medium for electrically connectingexternal circuits, such as printed circuit board (PCB) or other packagesubstrate boards. Accordingly, the package of the dies and theintegrated circuits is completed.

In order to connect the dies and the package substrate, wires and/orconductive bumps are often applied. In the Flip Chip InterconnectTechnology (FCIT), an array of conductive bumps is formed on the bodingpads of the dies. Then the dies are flipped so as to connect theconductive bumps to the contacts of the package substrate. Accordingly,the dies are electrically connected to the package substrate via theconductive bumps, and electrically connected to the external circuitsvia the internal circuits and the contacts of the package substrate.

FIG. 1 is a schematic cross sectional view of the structure of a priorart UBM layer. The passivation layer 104 is formed on the active surface102 of the chip 100. The passivation layer 104 covers the active surface102 of the chip 100. An opening 106 is formed in the passivation layer104 and exposes the top surface 112 of the bonding pad 110, serving as acontact window for subsequent bumping process. The UBM layer 120 and aconductive bump are formed on the bonding pad 110 by the bumpingprocess. The UBM layer 120 is formed between the top surface 112 of thebonding pad 110 and the conductive bump 130 for enhancing the adhesionbetween the bonding pad 110 and the conductive bump 130. The material ofthe conductive bump 130 can be a solder material such as Sn—Pb. Thespherical bump can be formed by a reflow process.

It is noted that the UBM layer is formed substantially conformally tothe structure of the opening 106, covering the top surface 112 of thebonding pad 110 and the surface surrounding the opening 106. Duringoperation of the chip 100, large currents flow through the UBM layer120, resulting in high current density at the region 108 close to thetransmission line 114. Due to current crowding in the region 108 of theUBM layer 120, metal atoms diffuse along the electron flowing directionat the lattice boundary of this region 108. This phenomenon is calledelectromigration. Electromigration causes losses of metal atoms of theUBM layer 120. This electromigration phenomenon is more serious at theportion which is close to the transmission line 114 than the otherportion which is away from the transmission line 114, which reduces thelife time of the chip 100.

SUMMARY OF INVENTION

Accordingly, the present invention is directed to an improved structureof an under bump metallurgic (UBM) layer. By increasing the thickness ofthe UBM layer which is close to the transmission line, the chip withsuch UBM layer becomes more reliable.

In order to achieve the object described above, the present inventiondiscloses a UBM layer which is adapted for a chip. The chip comprises atleast a bonding pad and a transmission line coupled thereto. Byincreasing the thickness of the UBM layer which is close to thetransmission line, losses of the UBM layer resulting fromelectromigration can be compensated by the increased thickness of theUBM layer. The life time of the chip is, therefore, enhanced.

According to one embodiment of the present invention, the UBM layercomprises a plurality of metal layers and at least one liner layer. Theliner layer is disposed among the metal layers and close to an end ofthe transmission line coupled to the bonding pad. In addition, the UBMlayer may comprise a plurality of metal layers of which portions closeto the transmission line are thicker than the other portions that areaway from the transmission line.

The above and other features of the present invention will be betterunderstood from the following detailed description of the preferredembodiments of the invention that is provided in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross sectional view of the structure of a priorart UBM layer.

FIG. 2 is a schematic cross sectional view of the structure of an underbump metal (UBM) layer according to one embodiment of the presentinvention.

DETAILED DESCRIPTION

FIG. 2 is a schematic cross sectional view of the structure of an underbump metal (UBM) layer according to one embodiment of the presentinvention. In this embodiment, the UBM layer 220 comprises, for example,a first metal layer 222, a liner layer 224, a second metal layer 226 anda third metal layer 228. The liner layer 224 is disposed among any twoadjacent metal layers. For example, the liner layer 224 can be disposedbetween the first metal layer 222 and the second metal layer 226, orbetween the second metal layer 226 and the third metal layer 228. Themetal layers 222, 226 and 228 may function as an adhesive layer, abarrier layer and a wetting layer, respectively. The metal layers can beformed by sequential deposition. A passivation layer 204 is formed onthe active surface 202 of the chip 200. The material of the passivationlayer 204 can be an organic passivation material or an inorganicpassivation material by a deposition process, for example. Thepassivation layer 204 covers the active surface 202 of the chip 200. Thepassivation layer 204 protects the transmission line 214 of the chip200. One end of the transmission line 214 is coupled to the bonding pad210. An opening 206 is formed in the passivation layer 204 and exposesthe top surface 212 of the bonding pad 210, serving as a contact windowfor the subsequent bumping process.

Referring to FIG. 2, the first metal layer 222 is formed substantiallyconformally to the profile of the opening 206 and covering the topsurface 212 of the bonding pad 210 and a portion of the surface of thepassivation layer surrounding the opening 206. According to thisembodiment the liner layer 224 is formed between the first and secondmetal layers 222, 226 and is disposed on a portion of the top surface ofthe first metal layer 222 which is close to the transmission line 214.The second metal layer 226 covers the other portion of the top surfaceof the first metal layer 222 which is away from the transmission line214. For the region of the UBM layer 220 which is close to thetransmission line 214, the liner layer 224 increases the total thicknessof the region. By increasing the thickness of the UBM layer 220 in theparticular region which is close to the transmission line 214, losses ofthe UBM layer 220 resulting from electromigration can be compensated.The life time of the chip 200 is, therefore, enhanced.

In this embodiment, the material of the first metal layer 222 can be Al,Ti, W or an alloy thereof, for example. The liner layer 224 can be madeof or include the same material as that of the first metal layer 222 forincreasing the thickness of the first metal layer 222 at the regionwhich is close to the transmission line 214. The material of the secondmetal layer 226 can be Cr, Ni or an alloy thereof, for example.According to another embodiment, the liner layer 224 can be made of orinclude the same material as that of the second metal layer 226. Theliner layer 224 increases the thickness of the second metal layer 226 atthe region which is close to the transmission line 214. The dispositionof the liner layer 224 can be arranged to increase the thickness of thefirst metal layer 222, the second metal layer 226, or the both.Accordingly, the portion of the UBM layer 220 close to the transmissionline 214 is thicker than the other portion of the UBM layer 220 that isfar away from the transmission line 214. By increasing the thickness ofthe UBM layer 220 at the region which is close to the transmission line214, losses of the UBM layer 220 due to electromigration occurring onthe UBM layer 220 can be compensated. The life time of the chip 200 is,therefore, enhanced.

In addition, the third metal layer can be, for example, a wetting layerwhich comprises material such as Ni, Au, Cu or an alloy thereof. Thethird metal layer 228 is formed over the second metal layer 226. Theconductive bump 230 adheres to the third metal layer 228 andelectrically connects to the bonding pad 210, as a conductive structureof the chip 200 for external connection. The liner layer 224 is notlimited to a single layer. One or more liner layers may be disposedwithin the UBM layer 220, For example the liner layer may be disposedbetween the second metal layer 226 and the third metal layer 228, on thethird metal layer 228, or under the first metal layer 222. As long asthe liner layer or liner layers enhance the thickness of the UBM layerat the region which is close to the transmission line.

Accordingly, the UBM layer of the present invention can be adapted for achip and to prevent electromigration occurring on the UBM layer. Theimproved UBM structure can be achieved by forming one or more linerlayers in the UBM layer or increasing the thickness of any metal layersof the UBM layer, at the region of the UBM layer which is close to thetransmission line. By increasing the thickness of the UBM layer at theregion which is close to the transmission line, losses of the UBM layerdue to electromigration occurring on the UBM layer can be compensated.The life time of the chip is, therefore, enhanced.

Although the present invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be constructed broadly to include other variants and embodimentsof the invention which may be made by those skilled in the field of thisart without departing from the scope and range of equivalents of theinvention.

1. An under bump metallurgic (UBM) layer adapted for a chip, the chipcomprising a bonding pad and a transmission line coupled thereto,wherein the UBM layer is formed over the bonding pad, wherein a portionof the UBM layer which is close to the transmission line is thicker thana portion of the UBM layer which is away from the transmission line. 2.The UBM layer of claim 1, wherein the UBM layer comprises a plurality ofmetal layers and at least one liner layer, the liner layer is formedamong the metal layers, and the liner layer is close to an end of thetransmission line which is connected to the bonding pad.
 3. The UBMlayer of claim 2, wherein the liner layer comprises a material the sameas a material of at least one of the metal layers.
 4. The UBM layer ofclaim 1, wherein a material of the UBM layer is selected from the groupconsisting of Al, Ti, W, Cr, Ni, Cu, Au, and an alloy thereof.
 5. TheUBM layer of claim 1, wherein the UBM layer comprises a plurality ofmetal layers, and at least one of the metal layers at a region which isclose to the transmission line is thicker than that which is away fromthe transmission line.
 6. A conductive structure over a bonding pad,adapted for a chip, the chip comprising a bonding pad and a transmissionline coupled thereto, the conductive structure comprising: an under bumpmetallurgical (UBM) layer formed over the bonding pad, a portion of theUBM layer which is close to the transmission line is thicker than aportion of the UBM layer which is away from the transmission line; and aconductive bump, wherein a bottom of the conductive bump is connected tothe UBM layer.
 7. The conductive structure of claim 6, wherein the UBMlayer comprises a plurality of metal layers and at least one linerlayer, the liner layer is formed among the metal layers, and the linerlayer is close to an end of the transmission line which is connected tothe bonding pad.
 8. The conductive structure of claim 7, wherein theliner layer comprises a material the same as a material of at least oneof the metal layers.
 9. The conductive structure of claim 6, wherein amaterial of the UBM layer is selected from the group consisting of Al,Ti, W, Cr, Ni, Cu, Au, and an alloy thereof.
 10. The conductivestructure of claim 6, wherein the UBM layer comprises a plurality ofmetal layers, and at least one of the metal layers at a region which isclose to the transmission line is thicker than that which is away fromthe transmission line.
 11. The conductive structure of claim 6, whereinthe conductive bump comprises a Sn—Pb alloy.